Prior art imaging devices have numerous structures to prevent the corruption of accumulated signal charge. These typically relate to the prevention of smear, blooming and crosstalk. Each of these effects requires different structures for correction or prevention.
Crosstalk is the phenomenon that describes photogenerated carriers, such as electrons, that diffuse from under the picture element (pixel) where they are created to other pixel areas where they are inadvertently collected. These diffusion electrons are generated in a field-free, or low field, region beneath the depleted region which forms the charge-collecting region of the pixel. The depletion region has a strong electric field that collects electrons. Crosstalk generally occurs in imagers that use the shift register, most often a charge-coupled device (CCD), to collect the photo-generated electrons.
Smear, typically, describes photogenerated electrons whose parent photons entered the image sensor through a charge-integrating region, such as a photodiode, but the photogenerated electrons are collected in an adjacent CCD. This is similar to crosstalk as discussed above, however, the term applies more to devices in which the shift registers are separated from the charge-integrating or collecting regions and where the shift registers run continuously. Hence, the crosstalk electrons are added to the charge packets in the shift register as they go by and the term smear is applied to this phenomenon.
Anti-blooming structures are those that are added to the image sensor to remove excess charge from the charge-integrating regions. The anti-blooming may be active or passive. The former requires a gate to be turned on and the latter works as soon as charge rises to a preset level. The goal of anti-blooming structures is to remove excess charge from a particular charge-collecting region before the excess charge enters adjacent charge collecting regions. Therefore, anti-blooming is a fundamentally different phenomenon than either cross-talk or smear.
While the three phenomenon of cross-talk, smear and blooming have different characteristics, the manner by which they have been controlled has involved similar elements within the prior art. Specifically, heavily positive doped regions, p+ regions, have been employed by image sensors that photogenerate electrons as the signal charge. In common with transistors and other integrated circuit elements, image sensors use isolation regions to separate charge packets, and/or current flow, in at least one dimension. For example, to separate parallel CCDs.
FIG. 1 illustrates a prior art sensor 5 formed in a heavily doped substrate 13 with a more lightly doped epitaxial layer 16 that forms junction 10 with substrate 13. The sensor 5 has p+ isolation regions 19 designed to have a lower potential than the charge storage regions 18 containing the charge packets. This is commonly done by implanting sufficient p-type dopants, usually boron. This is illustrated in the energy level diagram shown in FIG. 2. The underlying principle is that an electron is accelerated by a field in a direction that is opposite its current motion.
As discussed above, p+ regions are used to confine packets. A second use of a p+ region is to keep electrons in one region from reaching another region. This includes charge packet separation, however, it also includes using the p+ to keep diffusing electrons that are not part of a charge packet out of the region that contains the charge packet. This is one method to combat crosstalk. An example of this is to put p+ on almost all sides of a CCD shift register in an interline image sensor. This prevents smear by keeping electrons from other areas out of the CCD, and has been employed in prior art devices.
Matsunaga, Japanese Publication No. 58-62981, discusses smear reduction techniques within image sensors. Kumesawa et al also discusses smear reduction techniques within image sensors in IEEE Transactions On Electron Devices Vol. ED-32, No. 8 August 1985. However, these prior art structures are not useful in imagers where the CCD itself is the charge collecting and photosensitive region, because these structures lead to a reduced photo-response.
U.S. Pat. No. 5,051,798 to Kimura discloses a solid state image sensing device having an lateral overflow drain structure for controlling the blooming phenomenon that yields residual results towards smear reduction as a byproduct. However, this structure is not suitable for imagers which combine the charge collection and the charge transport functions in the same element.
Certain prior art devices teach smear and crosstalk reduction through the classic technique of increasing the depletion region associated with the charge collection element. Gardner et al, International Publication Number WO 92/21151, and Okada, discussed below, are recent examples.
Japanese Publication No. 4-316367 to Okada discloses an imaging device with a diode region that is enlarged for the reduction of smear. However, this is an interline type structure useful for these types of devices that incorporates a vertical overflow drain for anti-blooming. This structure does not address the concerns required for the reduction of cross talk in imagers without vertical overflow drains such as full frame imaging devices.
An example of a prior art device that is the subject of two patents that issued to Thenoz et al, U.S. Pat. No. 4,916,501 and U.S. Pat. No. 4,997,784, use a buried n+ region with a p+ region on the deeper side of the n+ region for anti-blooming. The role of the p+ region is to keep photogenerated electrons out of the n+ region. The p+ region reflects electrons by that mechanism, and may well increase cross-talk. In addition the n+ region may collect photogenerated electrons which lowers the photoresponse of this sensor.
Still another prior art device, Matsumoto as detailed in Japanese Publication No. 3-114260, describes the use of a buried p+ region to deter the electrons from below the p+ region from reaching the surface above the p+ region.
Another prior art device, disclosed by Jastrzebski and Levine in U.S. Pat. No. 4,481,522, utilizes electric fields created from doping gradients to drive electrons away from charge collecting regions. This has the result of reducing the photoresponse of the image sensor.
There is a single prior art disclosure Erhardt, in International Publication Number WO 90/12423, that employs surface p+ isolation regions on the periphery of the imager. However, no disclosure, or suggestion is made of using a deep p+ region to guide the flow of electrons.
It should be apparent from the foregoing discussion that there remains a need within the art for disclosures detailing the use of p+ dopant distributions to produce fields to guide electrons from neutral regions to charge collecting sites.